1. Field of the Invention
This invention relates generally to a premix fuel nozzle for use in a dual stage dual mode gas turbine combustor and more specifically to a premix fuel nozzle that does not contain a fuel circuit dedicated to support a pilot flame nor a fuel circuit dedicated to transfer a flame between combustor zones.
2. Description of Related Art
The U.S. Government has enacted requirements for lowering pollution emissions from gas turbine combustion engines, especially nitrogen oxide (NOx) and carbon monoxide (CO). These emissions are of particular concern for land based gas turbine engines that are used to generate electricity since these types of engines usually operate continuously and therefore emit steady amounts of NOx and CO. A variety of measures have been taken to reduce NOx and CO emissions including the use of catalysts, burning cleaner fuels such as natural gas, and improving combustion system efficiency. One of the more significant enhancements to land based gas turbine combustion technology has been the use of multiple combustor stages to lower emissions. An example of this technology is shown in FIG. 1 and discussed further in U.S. Pat. No. 4,292,801. FIG. 1 shows a dual stage dual mode combustor typically used in a gas turbine engine for generating electricity. Combustor 12 has first stage combustion chamber 25 and a second stage combustion chamber 26 interconnected by a throat region 27, as well as a plurality of diffusion type fuel nozzles 29. Depending on the mode of operation, combustion may occur in first stage combustion chamber 25, second stage combustion chamber 26, or both chambers. When combustion occurs in second chamber 26, the fuel injected from nozzles 29 mixes with air in chamber 25 prior to ignition in second chamber 26. As shown in FIG. 1, an identical fuel nozzle 29 is positioned proximate throat region 27 to aid in supporting combustion within second chamber 26. While the overall premixing effect in first chamber 25 serves to reduce NOx and CO emissions from this type combustor, further enhancements have been made to the centermost fuel nozzle since fuel and air from this fuel nozzle undergo minimal mixing prior to combustion.
A combined diffusion and premix fuel nozzle 31, which is shown in FIG. 2, has been used instead of the diffusion type fuel nozzle 29 shown proximate throat region 27 in FIG. 1. When utilized in a dual stage combustor, fuel nozzle 31 supports both the establishment of a pilot flame in second combustion chamber 26 through dedicated fuel circuit 33 as well as to transfer the flame from first combustion chamber 25 to second combustion chamber 26 through increased fuel flow to premix injectors 32. Although some mixing improvement was attained through premix injectors 32, by creating a longer distance over which to mix fuel with surrounding air, nozzle 31 still contained a dedicated fuel circuit 33 that did not mix with air prior to exiting nozzle 31 and combusting. This dedicated fuel circuit 33, while providing a stable pilot flame source rich in fuel, does not provide adequate mixing prior to combustion, which is required to reduce emissions. Therefore, elevated levels of NOx and CO emissions continue to occur with this nozzle design.
What is needed is a fuel nozzle configuration that is completely premixed, can establish a flame in a second combustion chamber of a dual stage dual mode combustor without a dedicated pilot fuel source, and move a flame from the first combustion chamber to the second combustion chamber utilizing existing fuel premix circuits. A fuel nozzle having this structure will not only reduce overall operating emissions, but will have a simpler design and reduce overall manufacturing time.